This invention relates generally to a method for applying a layer of gold metallization to a silicon substrate, and more particularly to a one step process for applying gold or gold alloy metallization to a silicon substrate.
In the fabrication of semiconductor devices the device function is implemented in a silicon wafer through process steps of diffusion, oxidation, and the like. A large number of devices are usually fabricated on a single silicon wafer; at the completion of the processing the wafer is divided into a plurality of individual devices and each of the devices is mounted in a package for protection, heat sinking, and electrical connection. The package is metal or has a metal portion to which the device is attached. The attachment is accomplished by soldering, gluing with an adhesive epoxy, or otherwise bonding between one surface of the die and the metal or metallized package portion. In one packaging technique which is particularly efficacious for small devices and for those requiring high reliability, the die is attached using a gold/silicon eutectic bonding process. In this technique the back surface of the silicon die is coated with gold and heated to form a gold/silicon eutectic. The die is then attached to a metallized package by an additional heating step which utilizes the gold/silicon eutectic as a solder. Additional layers of gold may be added over the alloyed layer and gold or gold alloy preforms may be used as well to facilitate the bonding.
While the foregoing technique of gold/silicon eutectic bonding is a preferred method in many applications, the implementation of the method in practice is fraught with difficulties. To insure adequate bonding between the silicon and the gold, the silicon wafer with the layer of gold thereon must be raised to a temperature in excess of the gold/silicon eutectic temperature of about 373.degree. C. At temperatures greater than 373.degree. C. the eutectic forms rapidly and silicon diffuses through the gold layer and gold diffuses into the silicon. One problem is that the gold is usually applied to a thin wafer, i.e., a wafer of only about 150 microns in thickness. Such a thin wafer is very fragile and the heating and cooling in addition to the handling itself is likely to cause the breakage of a high percentage of wafers. Also, if the silicon diffuses completely through the gold layer so that silicon atoms are exposed at the surface of the gold, these silicon atoms can subsequently be oxidized, either by heating during a subsequent assembly step or just on exposure to room ambient. The formation of oxidized silicon at the gold surface acts as a barrier to the complete wetting of the surface during attachment to the metal package portion. This results in the formation of voids in the die bond which can severely impact the reliability of the die bond. One solution to this problem has been to apply a layer of gold to the silicon wafer, heat that layer to form the gold/silicon eutectic, and then apply a second layer of gold which is not heated. The second layer of gold protects the underlying gold/silicon eutectic from the ambient and prevents oxidation of any silicon present there. This solution, however, requires additional processing steps as well as the use of additional gold. In addition, it is often difficult to achieve good adhesion between the two gold layers because of the presence at the interface of either oxidized silicon or contaminants. A further solution which has been attempted is to form the gold silicon eutectic and then, immediately before the bonding operation is to occur, to etch the surface of the eutectic to remove a thin surface layer and to expose a fresh surface free from oxidized silicon and having a lower silicon content. This solution also involves additional handling of fragile wafers, and is ineffective if the die from the wafer are not to be immediately assembled, because the exposed silicon in the fresh surface layer can oxidize during any lengthy delay.
In view of the difficulties encountered with the prior art processes involving multiple steps of evaporation or evaporation/heat or evaporation/heat/etch, a need existed for an improved one step metallization process. It is therefore an object of this invention to provide an improved process for applying gold metallization layers to the back of silicon wafers.
It is a further object of this invention to provide an improved process for the assembly of semiconductor devices.